1. Field of the Invention
The present invention relates to a high-power-output semiconductor laser device, and particularly to a high-power-output nitride semiconductor laser device that is unlikely to cause catastrophic optical damage (COD) in both end mirror parts of its optical cavity and a method of producing the same.
2. Description of the Background Art
A semiconductor laser device operating at high power output has a limited lifetime until it causes a malfunction. The most common malfunction in the high-power-output semiconductor laser device is catastrophic optical damage (COD) in both end mirror parts of the optical cavity.
A mirror part of an optical cavity of a semiconductor laser device includes a high density of non-radiative recombination centers which act as non-radiative recombination sites for carriers. Non-radiative recombination current generates heat. Besides, a relatively low density of carriers near the mirror part cause absorption of laser light. Heat generated by both the non-radiative recombination current and light absorption causes a temperature rise of the mirror part. When the semiconductor temperature rises, its energy band gap decreases. The band gap reduction acts to promote further light absorption. Such increased light absorption may cause a runaway process damaging the mirror part of the laser device under high-power-output operation, which may result in a malfunction of the laser device.
In order to alleviate such an output power limitation, various kinds of laser structures have been proposed. One example is known as a NAM (non-absorbing mirror) structure. In order to achieve this structure, the energy band gap of the active layer in a laser device is made larger in a region near the mirror than in other regions by utilizing diffusion of impurities, ion implantation of impurities, or epitaxial re-growth. A laser device having this NAM structure is able to operate at an output power that is greater by about one order of magnitude compared to conventional laser devices.
However, there are various limitations in such methods of producing the NAM structure as described in published documents. For example, both of the methods disclosed in U.S. Pat. Nos. 4,983,541 and U.S. Pat. No. 5,113,405 are inconvenient methods, each of which needs two-step epitaxial growth. While U.S. Pat. No. 5,395,793 discloses a method of utilizing low energy ion implantation so as to cause a distribution of defects inside a layer above a quantum well layer, it is necessary that part of the defects is diffused downward and mixed with the quantum well by heating subsequent to the ion implantation.